In the U.S., a significant portion of air pollution is caused by “mobile sources”, which include passenger cars, heavy duty vehicles, and motorcycles. Pollution caused by vehicles is the product of the combustion of hydrocarbon fuels (e.g. gasoline) to release the energy required to propel them. In ideal combustion, oxygen in the air would combine with all the hydrocarbons in gasoline to create only carbon dioxide and water as products.
This ideal combustion, however, never occurs. In reality, the combustion process generates other products, primarily carbon monoxide, various nitrogen oxides, collectively known as NOx, and unburned hydrocarbons (UHC). Carbon monoxide is a deadly, odorless, colorless, poisonous gas. Unburned hydrocarbons and nitrogen oxides are major causes of ground level ozone and smog in urban areas, both of which can cause breathing difficulties and lung damage. To reduce the emission of these pollutants, the U.S. Environmental Protection Agency (EPA) has regulated automotive emissions since its creation in 1970. The EPA dictates how much pollution automobiles and other mobile sources may emit, but manufacturing companies are free to choose how to achieve these limits. In practice this has led to catalytic converters being required for all cars sold and registered in the U.S.
The purpose of a catalytic converter is to remove harmful species from the exhaust stream by reacting them with other exhaust components to form more benign species. Important reactions performed include the reduction of NOx, which would otherwise contribute to the formation of photo-chemical smog and acid rain, the conversion of CO to CO2 and the catalytic combustion of unspent hydrocarbons that make it into the exhaust stream. Ideally, the treated exhaust contains only CO2, N2 and H2O after passing over the converter.
Currently “three-way catalysts” are the main technology used to control emissions from gasoline internal combustion engines. Three-way catalysts use a metallic or ceramic substrate to support a thin active coating typically incorporating alumina and combinations of the platinum group metals (PGMs), defined as platinum, palladium, and rhodium. Three-way catalysts oxidize hydrocarbons into carbon dioxide and water, carbon monoxide into carbon dioxide, and reduce nitrogen oxides into nitrogen and oxygen. The design parameters of a three-way catalyst can be adjusted to meet the required level of pollutant conversion associated with an application or regulation.
Catalytic converters are more effective at higher temperatures. Between 60% and 80% of all emissions emitted during the U.S. Federal Testing Protocol (FTP) occur during the first few minutes of operation, before the catalytic converter reaches its operating temperature (typically about 300° C. or more). To reach its operating temperature faster, the catalytic converter can be placed closer to the engine so it is exposed to higher temperatures more quickly after engine at startup. The noble metal catalyst in a three way catalyst is usually supported by a layer of γ-alumina due to its high surface area. However, at above 1050° C., γ-alumina can transform to α-alumina, which has a much lower surface area.
Despite their effectiveness in reducing emissions, the need to use PGMs as catalysts makes the production of three-way catalysts harmful to the environment. The use of these metals is the main drawback to the current catalytic converter design; the mining and processing of PGMs is damaging to the environment, which partially offsets the environmental benefits of using a converter. In order to extract one ounce of platinum, over seven tons of ore need to be processed. It is estimated that in South Africa, the world's largest platinum producer, up to 11 kg of coal are burned to extract enough platinum for one catalytic converter, which releases significant amounts of sulfur dioxide and carbon dioxide into the atmosphere. While PGM-based three-way catalysts are extremely effective in reducing emissions locally, when their life cycle is assessed and their global impact considered the result is less than stellar.
Apart from environmental considerations, there is a significant economic impact in using precious metals as catalysts. For example, platinum costs more than $1500 per ounce (circa 2011) and as much as half an ounce is needed for a typical automotive catalytic converter. The cost associated with these metals makes catalytic converters quite expensive relative to other components of the vehicle, which represents a significant added cost to car manufacturers.
Thus, it is desirable to provide a catalyst and a catalytic converter which reduces or eliminates the need for precious metals. It is also desirable to provide catalytic converter technology which can achieve “light off” temperature more quickly and/or can survive operation at higher temperatures. It is further desirable to provide a catalytic converter utilizing a catalyst which would cause considerably less environmental damage as well as provide a more cost effective alternative.